The incidence of ACL injuries in females has been reported to be two to eight times higher than that of males participating in the same sport, which has drawn increasing attention from investigators studying the underlying mechanisms of noncontact injuries. The overall objective of the proposed studies is to obtain quantitative data on potential gender-specific mechanisms of noncontact ACL injuries. The specific aims are: (1) To investigate gender differences in the in situ force and force distribution in the ACL in response to landing from a jump;(2) To investigate whether an ACL injury prevention training regimen would reduce the in situ force and alter the force distribution in the female ACL during landing from a jump;and (3) To investigate the correlation between the in situ force and strain in the ACL and anatomical parameters of the lower extremity. To achieve these aims, state-of-the-art technologies have recently been developed. A high-speed biplane fluoroscopy system will be used for collecting accurate knee kinematics in vivo from healthy volunteers landing from a jump. These knee kinematics will be reproduced on human cadaveric knees of the same gender using a high payload robotic/universal force-moment sensor (UFS) testing system to determine the in situ force and force distribution in the ACL. Simultaneously, the in vivo knee kinematics will be used as boundary conditions for subject-specific finite element models with a sophisticated constitutive model to determine the stress and strain distributions in the ACL.